The present invention relates to a receiver for receiving frequency shift keyed signals comprising a converter for sampling an input signal and mixing it with a signal having a frequency related to the sample frequency to obtain an intermediate signal, the receiver further comprises a detector for deriving a detected signal from the intermediate signal.
The present invention relates also to a telephone terminal using such a receiver and to a receiving method.
A receiver according to the preamble is known from U.S. Pat. No. 5,536,590. Such receivers find widespread use in all kind of equipment such as modems and telephone terminals.
In present day analog telephone systems, it is often required to send digital information from a telephone exchange to a subscriber. Examples of this digital information are e.g. Caller Identification information and information to be displayed on the screen of a screenphone. In the latter case, the digital information is transmitted according to the so-called ADSI standard (Advanced Digital Signaling Interface).
In order to be able to transmit digital information over the analog telephone network, Frequency Shift Keying (FSK) is used, in which the state of the digital signal is represented by a corresponding frequency. In telephone systems often binary FSK is used in which frequencies of 1200 and 2200 Hz represent the digital symbols.
In the known receiver for these FSK signals, the audio signal is sampled and subsequently mixed with a local oscillator which lies midway between the two frequencies of 1200 and 2200 Hz to obtain an intermediate signal. From the intermediate signal the detected signal is derived.
In order to simplify the receiver often there is a relation between the local oscillator frequency and the sample frequency, making it possible that the mixer arranged for mixing the local oscillator signal and the input signal is implemented digitally.
In the receiver known from the above mentioned US patent, the local oscillator frequency has a value of 1700 Hz. Because the sample frequency must be related to the local oscillator frequency, it has to be a multiple of 1700 Hz. In the known receiver this sampling frequency is 6800 Hz.
Often in telephone system the sample rate of the audio signals has to be at least equal to 8000 Hz to meet the requirements imposed by the sampling theorem, but preferably it is not chosen much larger in order to limit the complexity of the system. Due to the different sampling rates required in the system a sample rate converter would be required in order to derive a signal with a sample frequency of 6800 Hz from a signal with a sampling rate of 8000 Hz. This additional sample rate converter increases the complexity of the receiver substantially.
The object of the present invention is to provide a receiver according to the preamble having a reduced implementation complexity.
To achieve said purpose the receiver according to the present invention is characterized in that the frequency of the signal with which the input signal is mixed is unrelated to the frequencies corresponding to a logical state of the frequency shift keyed signal.
The present invention is based on the recognition that is not necessary that the frequency with which the input signal is mixed with, is the average of the frequencies corresponding to the different states of the frequency shift keyed signal. This recognition goes against the teaching according to a plurality of prior art documents. Besides the above mentioned US patent application the same teaching is disclosed e.g. in U.S. Pat. Nos. 4,475,219, 5,155,446 and UK Patent application No. 2 137 836. This recognition enables that the frequency with which the input signal is mixed is chosen independently from the frequencies representing the states of the FSK modulated signal.
An embodiment of the invention is characterized in that the converter is arranged for providing quadrature related signals representing the intermediate signals.
Using quadrature signals for representing the intermediate signal, it becomes very easy to determine the current phase of the intermediate signal. This facilitates an easy implementation of the detector for deriving a detected signal from the intermediate signal.
A further embodiment of the invention is characterized in that the frequency of the signal with which the input signal is mixed is equal to one fourth of the sample frequency of the intermediate signal.
This choice of the relation between sample frequency and frequency with which the input signal is mixed can lead to a substantially simplified convertor.
A further embodiment of the invention is characterized in that the convertor is arranged for deriving a first quadrature signal by multiplying the input signal cyclically with values proportional to +1,0,xe2x88x921,0 and in that the converter is arranged for deriving a second quadrature signal by multiplying the input signal cyclically with values proportional to 0,+1,0,xe2x88x921.
This embodiment leads to a very simple converter which does not require a multiplier for mixing the input signal. The quadrature signals can be obtained by performing at four subsequent sample instants the following operations. At the first sample instant, a sample of the input signal is passed unaltered to the output carrying the first quadrature signal, and the value 0 is passed to the output carrying the second quadrature signal. At the second sample instant, the value 0 is passed to the output carrying the first quadrature signal, and a sample of the input signal is passed unaltered to the output carrying the second quadrature signal. At the third sample instant, the inverted value of a sample of the input signal is passed to the output carrying the first quadrature signal, and value 0 is passed to the output carrying the second quadrature signal. At the fourth sample instant, the value 0 is passed to the output carrying the first quadrature signal, and the inverted value of a sample of the input signal is passed to the output carrying the second quadrature signal.
A further embodiment of the invention is characterized in that the detector comprises a phase detector for determining an approximation of the phase of the signal represented by the in-phase signal and the quadradure signal, and in that the detector comprises difference determining means for determining the change of said approximation of the phase as function of time.
By determining an approximation of the phase signal instead of the exact value of the phase the detector can be simplified. The logical state represented by the input signal can be easily determined from the difference between approximated phase values at different sample instants.